1. Field of the Invention
This patent relates generally to transmission systems. More specifically, the present invention relates to data transmission over signal paths with rapidly changing linear distortion, which is frequently found in radiated signals, and a method to correct therefore.
2. Description of Prior Art
One of the severe distortions that affect digital transmissions is multipath. Multipath is a linear distortion that is also known as echoes or ghosts. Multipath can be created by signals reflecting from buildings and impedance discontinuities in cables. With multipath, one or more copies of the original signal are added to the original signal, typically with delay and attenuation. On broadcast analog television pictures received via radiated transmissions, multipath appears as additional fainter images that are typically delayed relative to the image received via a direct path. On a digital transmission, severe multipath renders the data useless. Mild multipath increases the bit error rate (BER) in the presence of random noise or other additive impairments.
Other linear distortions also affect digital transmissions. Some of these distortions are non-flat frequency response and group delay. These distortions can occur because of imperfect filters and amplifier tut.
One prior art solution to correct for multipath is to employ a device called an adaptive equalizer. Adaptive equalizers are very well known in the art and are widely used. These devices work by summing a delayed version of the distorted received signal with the distorted received signal to cancel the echoes in a process called de-ghosting. The tap coefficients in the adaptive equalizer must be programmed to cancel the echoes. Programming can be assisted by using a special signal called a training, or reference, signal. The coefficients for the adaptive equalizer are computed by using the received reference signal and a stored version of the ideal reference signal. As intermediate steps, the frequency and impulse response of the channel are determined. The tap coefficients are computed as the reciprocal of the impulse response. The adaptive equalizer can also be programmed by using blind equalization techniques.
The problem with prior art adaptive equalizer methods of echo cancellation (or de-ghosting) is that the programming time is slow, so rapidly changing multipath can not be accurately corrected. To remedy this problem, much work has been done on fast algorithms. Typically increases in speed are accompanied by decreases in coefficient accuracy. Adaptive Filter Theory by Simon Haykin (published by Prentice Hall, 3rd edition) explains the theory of adaptive equalizers.
Echoes may change for many reasons. If the transmit tower is swaying the echoes may not be stationary. Likewise, if the transmitter location is moving relative to the receiver location, the echoes will vary. Moving reflective objects, such as vehicular traffic or pedestrian traffic, also vary the reflective nature of the transmission path. Microwave transmissions over water may be disturbed by dynamic reflections off of water waves. As wavelengths become shorter and motion is faster, the convergence rate of the adaptive equalizer needs to be faster.
Other signal transmission methods, such as code division multiple access (CDMA) and frequency modulation (FM) are used because they are resistant to multipath. Some of these systems are less than optimal for other reasons such as hardware complexity or bandwidth efficiency.
Transmission of digital information is commonly done with blocks of data. It is a common practice to use a linear code for purposes of forward error correction (FEC), such as a Reed-Solomon code, whereby some percentage of erred symbols can be corrected by the code. Digital Communications Fundamentals and Applications by Bernard Sklar (published by Prentice Hall) explains the basics of practical digital communications including linear codes.
OFDM (orthogonal frequency division multiplexing) signals are sent in blocks and are comprised of many harmonics that are orthogonal to each other by virtue of their integer relationship to a fundamental frequency. By varying the phase and the magnitude of the harmonics, information can be transmitted while preserving the orthogonality between each of the harmonics. Echoes also affect OFDM signals. The effect of the echo can be corrected by using a guard interval in the time domain, provided the delay of the echo is shorter than the duration of the guard interval. Each received frequency coefficient may then be corrected by a single complex multiplication that corrects the magnitude and phase. The use of a training, or pilot, signal assists in the determination of the correct complex multiplication coefficient to use on each coefficient.
The European community plans to use OFDM modulation for terrestrial broadcast of advanced television signals. In the United States OFDM is being used to carry data over the hostile environment of cable television upstream plant. "Using Orthogonal Frequency Division Multiplexing in the Vertical Interval of an NTSC TV Transmission" by M. Chelehemal and T. Williams (published in 1995 Proceedings of the NAB) is one of many papers explaining OFDM. OFDM may be used at baseband, or it may be transported by an RF (radio frequency) or microwave carrier. It can be modulated onto the carrier using a QAM (quadrature amplitude modulation) system or a VSB (vestigial sideband) system.
Another method to solve the rapidly changing multipath problem is to embed a training signal in the burst or packet transmission. The GSM cellular phone system, which is widely employed in Europe, uses this method. Embedding a training signal in each voice packet transmitted reduces the amount of data that can be sent.
Characterizing channels using frequency domain techniques is known in the art Likewise, using OFDM signals with fixed data as reference signals is also known in the art since OFDM signals that have constant spectral energy make good reference signals.